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Feline Infectious Anemia
By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com

(also called feline hemotropic mycoplasmosis, infection by Hemobartonella felis, or infection by Mycoplasma haemophilus)

Parasitic organisms survive by attaching themselves to a host and using the host's body to thrive, generally at the host's expense. Parasites find themselves protected from the harsh temperature and moisture changes of the external world when they live within the rich, warm body of their host. The parasites this article concerns is a bacterium that attaches itself to the red blood cell membranes of its host, happily riding around, feeding and reproducing until the host's immune system sees it and begins destroying red blood cells in an attempt to remove it.

Hemobartonella felis (newly renamed Mycoplasma Haemophilus)

The agent of what has traditionally been called feline infectious anemia is an organism called (until recently) Hemobartonella fe lis. This creature is technically a bacterium but it's a member of a group of bacteria called mycoplasma. Mycoplasmas are different from other bacteria because they do not have a cell wall surrounding and protecting their microscopic bodies. They cannot be cultured in the lab like normal bacteria because they require living hosts.

Hemobartonella felis was first discovered in Africa in 1942 but it was not recognized as a mycoplasma until gene sequencing was possible recently. This has led to renaming Hemobartonella felis to Mycoplasma haemophilus, though after decades of celebrity we shall see if they new name becomes commonly used. Complicating matters further, gene sequencing has revealed a second species, previously thought to be another variant of Hemobartonella felis. This one, which is smaller, has been named Candidatus M haemominutum. This smaller mycoplasma seems to only be a problem for cats infected with the feline leukemia virus. The discussion that follows largely concerns Mycoplasma haemophilus.

The term feline infectious anemia has also recently felt to be inaccurate as there are many infectious organisms that might cause an anemia (lack of red blood cells). For this reason the disease itself has been re-named feline hemotropic mycoplasmosis, which literally means a blood infection of mycoplasma organisms in cats.

What Happens to the Infected Cat?

The infected cat's immune system is busy coating infected red blood cells with antibodies. Coated red blood cells are removed from the circulation by the spleen. This kills the organism, and the iron is harvested and recycled into new red blood cells. The problem is that so many red blood cells are being destroyed that the cat becomes anemic.

The infected sick cat is pale (sometimes even jaundiced) and weak. Anemic cats often eat dirt or litter in an attempt to consume iron. They may have a fever. The initial blood tests show not just red cell loss but a very responsive bone marrow (the source of new red blood cells), which means that the cat knows it is losing red cells and is trying to make more as quickly as possible to keep up. Cats with concurrent feline leukemia virus infection tend to have more severe anemias as the virus does not permit the bone marrow to respond.

When a cat is newly infected, it can take up to one month before adequate numbers of parasites are present to actually make the cat sick. Mortality is highest during the month following this initial stage. If the cat recovers, it becomes a carrier, though stress can re-activate the infection.

How is Diagnosis Confirmed?

Confirmation of diagnosis has been problematic since the discovery of the organism. Because it lacks a cell wall, Mycoplasma haemophilus cannot be cultured, which means one cannot simply culture a blood sample and isolate the organism the way one might isolate the organism causing something like a urinary tract infection.

Most reference labs scan all feline blood samples under the microscope looking for the characteristic appearance of infected red blood cells (see graphic at the top of the page). Unfortunately, the number of organisms cycles in a matter of hours such that the number of infected cells can change from 90% to 1% in a matter of 3 hours. This makes it very easy to miss infected cells, even in a grossly infected cat.

Luckily, PCR technology has made diagnosis easier, though carrier cats can still slip by. Unless organisms are actually seen on a blood smear, the PCR test is the type of test to request.

What Cats are at Risk?

The cats at highest risk are those that roam outside in the spring and summer (obviously these cats have the highest risk for flea infestation). Cats that are statistically likely to be infected are male cats younger than age 4 to 6 years, with a history of cat fights, and incomplete vaccination histories (in short, cats who've had somewhat casual care). Infection with the feline leukemia virus is also a factor in diagnosis. This may be because this immune-suppressive virus allows proliferation of the organism that is not possible in normal hosts or perhaps the anemia associated with the virus directly leads to a sicker cat who is thus more likely to see the veterinarian and be tested. Making matters worse, the presence of the mycoplasma seems to enhance the ability of the feline leukemia virus to create bone marrow cancers.

An abnormal immune system is absolutely not a necessity in infection with hemotropic mycoplasmas; normal cats are infected as well. Further, infection with feline immunodeficiency virus does not enhance the severity of hemotropic mycoplasma infection.

Blood sucking parasites such as fleas, ticks, lice, and mosquitoes are the leading candidates for spread of the organism. This makes flea control paramount in protection.

Cats can become infected by blood transfusion, though animal blood banks routinely screen donors so this is an unlikely route.

Infected mother cats appear to be able to infect their unborn or newly born kittens. Oral transmission is definitely possible.

Treatment

If hemotropic mycoplasma infection is suspected, initiating treatment is probably a good idea as treatment is much easier than diagnosis. All mycoplasma infections are susceptible to the use of tetracycline. In cats, the derivative doxycycline tends to be most easily dosed as it comes in an oral suspension. Tablets must be used with caution as they can stick in a cat's esophagus, cause irritation, and scarring. The quinolone class of antibiotics (enrofloxacin , etc.) is also effective against hemotropic mycoplasmas. Three weeks of medication is needed to adequately suppress the organism.

Killing the mycoplasma is only part of the therapy, however; it's the host's own immune system that is removing the red blood cells and this must be stopped. Prednisone or a similar steroid hormone is typically used to suppress this part of the immune system so that the red blood cells are not removed as quickly. Very sick cats will probably require blood transfusions to get through the brunt of the infection. Happily, prognosis is fair if the diagnosis is made in time as cats generally respond well and quickly to treatment.

Carrier cats are generally not treated.

Can Dogs be Infected?

There is an organism previously called Hemobartonella canis (now renamed Mycoplasma haemocanis). It's not generally considered to be a problem except in dogs who have lost their spleens and thus cannot effectively remove infected red blood cells. Gene sequencing suggests that Mycoplasma haemophilus may be able to disguise itself slightly when it lives in a dog's body. Blood from infected dogs, however, will not infect cats. It's not clear at this time what the relationship is between these two types of mycoplasma, but it appears that cats cannot infect dogs and dogs cannot infect cats. There is a hemotropic mycoplasma disease in dogs but it is separate from the kind in cats.

Date Published: 7/2/2004 11:07:00 AM

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